1. Technical Field
This invention relates to a countercurrent process for the continuous esterification of C1-22 (fatty) acids with C1-10 monoalkanols, C2-5 di- or trialkanols or mixtures thereof in the liquid phase in the presence of heterogeneous catalysts.
2. Background Art
Processes for the continuous esterification of fatty acids are known, cf. H. Stage, Chemiker-Ztg./Chem. Apparatur 87, No. 18, 661-666 (1963). This publication describes the esterification of fatty acids with methanol and n-butanol in multiple-section reaction columns with vapor baffle plates operated at normal pressure. However, the continuous esterification of fatty acids on an industrial scale is normally carried out in several stages in plate columns under pressures of 5 to 30 bar (cf. DE 2503195 C).
EP 0 334 154 B1 (Henkel) describes a countercurrent process for the continuous homogeneously catalyzed esterification of fatty acids with alkanols in the liquid phase in a reaction column, the catalysts and fatty acids being introduced onto the uppermost plate and the alkanols onto the lowermost plate and being reacted at a head pressure of the reaction column of 200 to 900 hPa. This process is said to avoid unwanted secondary reactions such as, for example, the dehydration of branched monoalkanols. In addition, in this homogeneously catalyzed process, the homogeneous catalyst has to be removed at the end of the reaction. This results in fairly high catalyst losses and also losses of product.
WO 90/11114 (Henkel) describes a continuous process for conducting a heterogeneously catalyzed reaction.
In discontinuous heterogeneously catalyzed processes, the solid catalyst is generally size-reduced by stirrers during its direct introduction into the reactor and has to be filtered off after the reaction. Now, WO 90/11114 describes a process in which the educts are premixed in a reactor and subsequently introduced by pumps into an external catalyst container. The reaction mixture then flows through a thin-layer or falling-film evaporator. The progress of the reaction is determined by continuous monitoring, for example of the acid value. A disadvantage of this process is that it is a discontinuous process. According to E. Fitzer, Technische Chemie, 4th Edition (1995), disadvantages of discontinuous processes are the dead times occurring during filling, emptying, heating and cooling, relatively high energy costs and relatively high labor costs.
EP 0 474 996 A1 (Hüls) describes a process for the production of esters from alcohols and acids by liquid-phase equilibrium reactions on ion exchangers. The reaction is carried out in a preliminary reactor and a rectification column with additional external reactors. On account of the severe thermal stressing of the ion exchanger in a reaction-type distillation column and the difficulties involved in separation by distillation in the presence of ion exchangers, the esterification and rectification have to be carried out in separate spaces. However, these disadvantages are reliably overcome by the process according to the invention.
The problem addressed by the present invention was to provide an economic process for the esterification of (fatty) acids with alkanols which would be a continuous, heterogeneously catalyzed process free from the disadvantages mentioned above. In addition, the color and odor of the products would be improved by less exposure to heat during production.